Start/charge amps
Thread Starter
Melting Slicks
Joined: Jul 2013
Posts: 2,012
Likes: 23
From: Anaheim CA
Start/charge amps
No problem, just idle curiosity. I do a lot of short hops, in the 3-5 mile round trip range...for better or worse, just about everything I need is close to the house. So I got to wondering how long the engine needs to run to put back into the battery what came out during each start. I'm assuming, as a rough approximation, that start amps X cranking time has to equal charge amps X run time. Easy enough to solve, except I don't have a good feel for the typical values for 'amps' on either side of the equation.
My question is, how many amps does the starter typically draw and how many amps get delivered to the battery in charge mode?
My question is, how many amps does the starter typically draw and how many amps get delivered to the battery in charge mode?
Le Mans Master
Joined: Sep 2013
Posts: 8,900
Likes: 1,074
From: Wisconsin
No problem, just idle curiosity. I do a lot of short hops, in the 3-5 mile round trip range...for better or worse, just about everything I need is close to the house. So I got to wondering how long the engine needs to run to put back into the battery what came out during each start. I'm assuming, as a rough approximation, that start amps X cranking time has to equal charge amps X run time. Easy enough to solve, except I don't have a good feel for the typical values for 'amps' on either side of the equation.
My question is, how many amps does the starter typically draw and how many amps get delivered to the battery in charge mode?
My question is, how many amps does the starter typically draw and how many amps get delivered to the battery in charge mode?
Instructor
Joined: Nov 2013
Posts: 120
Likes: 7
From: overland park kansas
charge
No problem, just idle curiosity. I do a lot of short hops, in the 3-5 mile round trip range...for better or worse, just about everything I need is close to the house. So I got to wondering how long the engine needs to run to put back into the battery what came out during each start. I'm assuming, as a rough approximation, that start amps X cranking time has to equal charge amps X run time. Easy enough to solve, except I don't have a good feel for the typical values for 'amps' on either side of the equation.
My question is, how many amps does the starter typically draw and how many amps get delivered to the battery in charge mode?
My question is, how many amps does the starter typically draw and how many amps get delivered to the battery in charge mode?
Thread Starter
Melting Slicks
Joined: Jul 2013
Posts: 2,012
Likes: 23
From: Anaheim CA
My rough-cut back-of-the-envelope worst-case analysis came out something like this:
Cranking draw is something on the order of 200A, assuming a 50% efficiency 1.5HP starter motor. Figure 2 seconds of cranking time, for a total of 400 amp-seconds from the battery.
Typical battery chargers operate in the 8-10A range in quick-charge mode, let's figure the charging system in the car is optimized for battery life and use half that: 4A. So it takes 100 seconds of run-time to pull the battery back up to where is started, and I doubt I could come up with a more conservative set of assumptions.
Sounds like it's not a problem. I'd still like some authoritative numbers for start and charge current though.
(Note: I doubt the starter draws much more than 100A, it's probably something under 1HP. Also, if it cranks for a full two seconds before it starts, something is very wrong. I may be way off on charge current though.)
Cranking draw is something on the order of 200A, assuming a 50% efficiency 1.5HP starter motor. Figure 2 seconds of cranking time, for a total of 400 amp-seconds from the battery.
Typical battery chargers operate in the 8-10A range in quick-charge mode, let's figure the charging system in the car is optimized for battery life and use half that: 4A. So it takes 100 seconds of run-time to pull the battery back up to where is started, and I doubt I could come up with a more conservative set of assumptions.
Sounds like it's not a problem. I'd still like some authoritative numbers for start and charge current though.
(Note: I doubt the starter draws much more than 100A, it's probably something under 1HP. Also, if it cranks for a full two seconds before it starts, something is very wrong. I may be way off on charge current though.)
Last edited by Lee DeRaud; Jul 9, 2014 at 01:43 PM.
Race Director
Joined: Apr 2007
Posts: 11,150
Likes: 890
From: South Western Ontario
Ya, you're very conservative. The starter will draw below 200A and the alternator does not limit the battery charging current to only 4A. You could basically consider the alternator as a 14.4V source with a 110A or 140A current limit. At idle, the alternator might produce about 75% of it's rated current and the cars electrical loads are using some of the alternator output current so the possible battery charging current is less than the maximum alternator rating.
